This invention relates to metal oxide varistors and, in particular, to lithium-doped zinc oxide based varistors with controllable breakdown voltage and capacitance.
In general, a metal oxide varistor comprises a zinc oxide (ZnO) based ceramic semiconductor device with a highly nonlinear current-voltage relationship which may be represented by the equation I=(V/C).sup..alpha., where V is the voltage between two points separated by the varistor material, I is the current flowing between the points, C is a constant, and .alpha. is a measure of device nonlinearity. If .alpha.=1, the device exhibits ohmic properties. For values of .alpha. greater than 1 (typically 20-50 or more for ZnO based varistors), the voltage-current characteristics are similar to those exhibited by back-to-back connected Zener diodes. Varistors, however, have much greater voltage, current, and energy-handling capabilities. If the voltage applied to the varistor is less than the varistor breakdown voltage, only a small leakage current will flow between the electrodes and the device is essentially an insulator having a resistance of many megohms. However, if the applied voltage is greater than the varistor breakdown voltage, the varistor resistance drops to low values permitting large currents to flow through the varistor. Under varistor breakdown conditions, the current through the varistor varies greatly for small changes in applied voltage so that the voltage across the varistor is effectively limited to a narrow range of values. The voltage limiting or clamping action is enhanced at higher values of .alpha..
Metal oxide varistors have been widely employed as surge arresters for protecting electrical equipment from transients on AC power lines created by lightning strikes or switching of electrical apparatus. Such applications require the use of varistors having breakdown voltages slightly greater than the maximum input voltage of the system to be protected. Thus, for example, a typical system powered from 170 volts peak voltage (120 volts rms) AC power mains would require the use of a varistor having a breakdown voltage somewhat greater than 170 volts.
Varistor device behavior may be approximately modeled by a variable resistor in parallel with a capacitor. The parasitic capacitance modeled by the capacitor is an intrinsic property associated with the particular varistor composition, and is generally undesirable as it may affect varistor performance in surge-protective or switching applications, for example. In typical surge-arrester applications, the varistor is subjected to a continuously applied voltage. Although the applied voltage is lower than the varistor breakdown voltage, an undesirable current, due predominantly to the parasitic capacitance, flows through the varistor. In high frequency circuits this current flow may be large enough to affect normal operation of the circuit.
Another capacitance-related problem (described in greater detail in U.S. Pat. No. 4,276,578, issued to L. M. Levinson, and assigned to the same assignee as the present invention) arises in surge-arrester devices made up of stacked metal oxide varistors. In such devices, each varistor in the stack has in addition to the parasitic capacitance associated therewith, a coupling capacitance to ground. As a result of the combined effect of the parasitic and ground capacitance, particularly ground capacitance, a larger current flows through the top varistors (those nearest the line) in the stack since these varistors also pass the capacitive ground currents which flow through the lower varistors. The upper varistors therefore are required to dissipate greater power, resulting in higher operating temperature, inferior stability, and concomittantly shorter useful life due to premature failure. In conventional systems, discrete, low dissipation capacitors are connected in parallel with the varistors to achieve a more uniform voltage and power distribution throughout the stacked varistors. Use of capacitors with graded intrinsic capacitances, as described in the aforementioned patent, is a more effective solution.
Varistor elements may also be used as switching elements for multiplexing, for example, liquid crystal displays. In such applications, the parasitic capacitance is also a problem, since it appears in series with the capacitance of the liquid crystal material, forming a capacitive voltage divider. A lower electric field than would otherwise be available is thus used to maintain the liquid crystal material in its active state. Additionally, if the varistor capacitance is too high, nonselected elements in the liquid crystal array may be inadvertently activated by pulses applied to the display. A more detailed description of multiplexing liquid crystal displays using varistors appears in U.S. Pat. No. 4,223,603 issued to D. E. Castleberry and in application Ser. No. 233,423 filed Apr. 11, 1981 by L. M. Levinson, both assigned to the same assignee as the present invention.
From the foregoing the importance and desirability of reducing varistor capacitance is apparent. Aforementioned U.S. Pat. No. 4,276,578 discloses the inclusion of antimony oxide (Sb.sub.2 O.sub.3) in the varistor for the purpose of decreasing intrinsic capacitance. The present invention provides varistors with high breakdown voltage and low capacitance by controlled diffusion of lithium into conventional zinc oxide varistor material.